Adjust typing of ApolloServerTestClient to prevent linter error with createTestClient

[lauriharpf]

Adjusts typing for createTestClient in apollo-server-test so that @typescript-eslint/unbound-method linter error is no longer reported by typescript-eslint.

Closes #4724 . Repo https://github.com/lauriharpf/reproduce-create-test-client-linting-issue has an example that reproduces the issue. Some additional background in Stackoverflow: https://stackoverflow.com/questions/62864908/how-to-prevent-typescript-eslint-unbound-method-errors-on-non-class-typescrip

[apollo-cla]

@lauriharpf: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[glasser]

As described in #4724 (comment) I think the whole apollo-server-testing package is unnecessary. Rather than changing its typing I'd prefer to just stop pointing people at it as described in #4952.